I originally thought blocking access could be due to a licensing thing, given that Apple and Intel collaborated on it [0]. However I found [1] stating that the licensing fees and royalties were waved in order to ensure larger market adoption.

[0] https://en.m.wikipedia.org/wiki/Thunderbolt_(interface) [1] http://appleinsider.com/articles/17/05/24/intel-making-thund...